Rainfall intensification increases nitrate leaching from tilled but not no-till cropping systems in the U.S. Midwest

Abstract

As global surface temperatures rise, the percentage of total precipitation that falls in extreme events is increasing in many areas (“rainfall intensification”), including the U.S. Midwest, a major agricultural region. While it is well known that losses of nitrogen (N) fertilizers applied in excess of crop N demand have consequences for non-agricultural ecosystems, the effects of rainfall intensification on N losses from agricultural fields are uncertain. We conducted a 234-day field experiment in which we evaluated the effects of rainfall intensification on N leaching, soil inorganic N pools, soil N transformations, and crop N content in replicated tilled and no-till row crop systems of the upper Midwest. Under rainfall exclusion shelters we exposed 5 × 5 m plots to a control rainfall treatment with relatively small, frequent rainfall events historically typical of the region, and an intensified rainfall treatment with the same total rainfall added in larger, less frequent events. Although rainfall intensification increased modeled water percolation to 1.2 m in both tilled and no-till systems, as reported previously, it increased nitrate leaching only in tilled systems. Extractable soil nitrate concentrations throughout the experiment were on average 32 % higher in surface soils exposed to intensified rainfall compared to control rainfall regardless of tillage management. In-situ net N mineralization and nitrification rates measured during a two-week period in summer showed no significant differences between rainfall or tillage treatments. Inorganic N pools (0–1.2 m depth) were 43 % greater in no-till soils compared to tilled soils and were unaffected by rainfall intensification; crop N concentrations and total N were likewise unaffected. Our results suggest that rainfall intensification in tilled cropping systems will increase N leaching to groundwater, with consequent economic and environmental harm. No-till management, however, may buffer systems against the effects of intensification on nitrate loss.